If you pull the recoil starter, but it has little or no resistance, but the engine does turn over, then your engine has little or no compression. This can be caused by several factors...a loose glow plug, a loose cylinder head, or a loose piston/cylinder fit.

Check the glow plug and carefully tighten it if it's loose.

Check the bolts that hold the cylinder head to the engine. If a head bolt is loose, you'll have to loosen all of them and then carefully tighten them a bit at a time, doing one bolt, then the one opposite it, and then the next bolt, and so on. Try to tighten them as evenly as possible. If the engine is hot or warm, let it cool down to room temperature before you tighten the bolts. If you tighten them when the engine is warm or hot, they'll just come loose again.

If the plug is tight and the cylinder head bolts are tight, then you have a bad piston/cylinder fit. This is usually caused by overheating, running with a fuel that has too little oil, running the engine with a too-lean needle setting, or the ingestion of dirt. The only way to correct this condition is to send the engine to the service center for repair.